This invention relates to speed control in a robot and, more particularly, to a transfer of power among plural motors driving a common joint of the robot to equalize travel of the motors.
Robots are being employed with increasing frequency for positioning a large variety of loads in industrial processes. Both speed and precision are desirable qualities in robotic tasks, the speed enabling more efficient operation and the precision enabling the robots to perform operations requiring highly precise movements.
Robots are composed of one or more members which are movable relative to a frame of reference. A member may translate, pivot to rotate relative to another of the members. Some form of prime mover, such as an electric motor, is associated with each member for providing the movement. Each movable member, including its prime mover, is frequently referred to as a joint irrespective of the type of motion which the member may undergo. A load to be positioned by the robot is held by an end effector, or gripper, extending from one of the joints.
A situation of particular interest is the case of a robot wherein a joint translates sideways as in a robot structured with joints arranged in a cartesian coordinate system. For example, a joint disposed along the X-axis may be supported by a joint which translates along the Y-axis. The X-axis joint, in turn, may support a Z-axis joint which terminates in a gripper for carrying a load. For precise positioning of the load, it is desireable that both ends of the X-axis joint move in synchronism.
A problem arises in that such synchronism has been attained by a massive interconnecting structure which minimizes bending of the X-axis joint. The use of such heavy structure is inefficient in energy consumption and increases the cost of the robot. Also, the increased weight of the joint necessitates a larger drive motor. Larger electric motors in joints are disadvantageous because of slower response times which militate against desireable rapid precise movements of a robot.
One attempt to solve the foregoing problem is the use of two electric motors, one at each end of the X-axis joint. Such a configuration can be obtained by the use of two Y-axis joints disposed at opposite ends of the X-axis joint, and supporting the X-axis joint. The two motors are advantageous in that they are smaller and have faster response times than the foregoing larger motor. However, the dynamic loading of the two motors varies during a translation of the Z-axis joint along the X-axis joint. A problem still remains in that presently available motor control systems do not compensate for changing dynamic inertial loads to provide adequate synchronism between the two motors. As a result bending of the X-axis joint may occur with a possible binding in the drives and bearings of the two Y-axis joints.